Fine carbon fibers, as referred to in the present invention, are typically produced through a vapor phase method comprising thermal decomposition of hydrocarbon (e.g., disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 7-150419, 5-321039, 60-215816, and 61-70014 and Japanese Examined Patent Publication (kokoku) Nos. 5-36521 and 3-61768). The thus-produced carbon fibers typically have a diameter of approximately 0.01-5 μm. If the diameter is 0.01 μm or more, the fine carbon fibers include carbon nano-tubes and carbon nano-fibers having a concentric or tree-annual-ring structure similar to the carbon fibers grown by the vapor phase method.
There has been proposed use of fine carbon fibers as a filler for metals, resins, ceramics, etc. Of these, fine carbon fibers are particularly proposed as a filler for batteries, because the development of portable apparatuses such as small-sized cellular phones, video cameras, and notebook-type personal computers has been remarkable, and the demand for a small-sized secondary battery including an Li ion secondary battery (Li battery) serving as a power source therefor has increased drastically.
Examples of typical carbonaceous materials for a negative electrode of an Li battery include hard carbon pitch, meso-phase carbon microbeads (MCMB), meso-phase pitch carbon fiber (MPCF), artificial graphite, coke, and naturally occurring graphite. Furthermore, the addition of carbon fibers produced from pitch or vapor-grown carbon fibers to these negative electrode materials has also been proposed, while carbonaceous materials such as graphite micropowder and carbon black are incorporated into a positive electrode as electric conductivity-providing agents.
In a negative electrode of an Li battery, intercalation and deintercalation of lithium ions occur during charge and discharge processes. Graphite, having a layered structure, easily undergoes intercalation wherein a reactant (e.g., Li) is inserted into an interlayer void to thereby expand the void (intercalation). The reaction product containing a compound inserted in the interlayer void is called a graphite intercalation compound. The intercalation compound easily releases the intercalated reactant (deintercalation), to thereby transform itself into graphite; i.e., the original material. Fine carbon fibers, which is a material having excellent electric and thermal conductivity and an intercalating property, do not lower the capacity of the battery, and thus addition thereof is of interest as an additive in a negative electrode material.
The enhancing of the intercalating property of the negative electrode is essential for enhancing the capacity of an Li battery. In general, in order to enhance the intercalating property, the degree of graphitization; i.e., crystallinity, of carbonaceous materials including fine carbon fibers must be enhanced.
A negative electrode of a lead secondary battery per se comprises a compound having poor electric conductivity, and carbonaceous materials such as carbon black, graphite microparticles and carbon fiber may optionally be added thereto to enhance the conductivity of the negative electrode. Such carbonaceous materials desirably have high electric conductivity and crystallinity. In order to enhance the crystallinity of such carbonaceous materials, the materials are typically treated at high temperature in order to effect graphitization.
Meanwhile, fine carbon fibers having a small average fiber diameter, particularly 1 μm or less, have a low bulk density and provide an insufficient filling density. Thus, when fine carbon fibers are added, to an electrode, in a large amount, the density of the electrode decreases. Therefore, fine carbon fibers are typically added in an amount of 20 mass % or less, preferably 10 mass % or less. From this point of view, fine carbon fibers are considered not to impart a remarkable effect commensurate with addition thereof even when the crystallinity of the fibers is enhanced. Therefore, no investigation other than treatment at high temperature has been carried out so as to enhance the crystallinity of fine carbon fibers. Thus, conventionally used fine carbon fibers have insufficient crystallinity, and the distance between two crystal layers represented by d002 is larger than 0.3385 nm.
To meet with demand for increased capacity, electrode materials with a lower electric resistance are sought to give a large electric current charge-discharge capacity.
Addition of various conductivity-imparting materials are studied to lower the resistance of electrodes, in which it is known that a filler of a fibrous material particularly of vapor grown carbon fibers is effective. The reasons thereof include:
1) fine fibrous materials have an aspect ratio of 100 or more allowing a long conducting path;
2) vapor-grown carbon fibers have a high crystallinity to give a high conductivity; and
3) vapor-grown carbon fibers have a charge-discharge capacity to thereby prevent reducing the capacity of a Li battery when added.
However, commercially available fine fibrous materials having a diameter of 1 μm or less have an upper limit of the conductivity of 0.01 Ω·cm as the powder resistance when measured at a density of 0.8 g/cm3 and there are no available fine fibrous materials having a conductivity higher than this.
Recently, the crystallinity of a negative electrode material has been enhanced to thereby improve the charge-discharge capacity of a battery. This trend forces an additive other than a negative electrode material to have a high discharge capacity. Therefore, the crystallinity of carbonaceous material serving as an additive for a negative electrode must be enhanced.
Considering the above requisite to enhance the crystallinity of fine carbon fibers, the inventors have been investigated heat treatment up to 3200° C. to enhance the crystallinity.
However, even when fine carbon fibers (vapor-grown carbon fibers) having diameters of the order of about 0.15 μm are heated to 3000° C. or higher, the lattice constant of the interlayer distance d002 cannot be reduced to less than 0.3385 nm.
Simultaneously, the conductivity had an upper limit of 0.01 Ω·cm as the powder resistance when measured at a density of 0.8 g/cm3. Accordingly, fine carbon fibers having a higher crystallinity and a lower resistance are sought.
The supposed reason for the above failure is that vapor-grown carbon fibers have a considerably small fiber diameter and a unique structure wherein the core of the concentrically grown crystalline fiber comprises a hollow or amorphous portion. In addition, when the fiber diameter is as small as 1 μm or less, maintenance of a roll-shape structure of a carbon hexagonal network plane becomes more difficult with decreasing distance between the structure and the center of the fiber, to thereby make crystallization difficult. Therefore, the value of d002 depends on the diameter of the fiber. For example, the limitations on the interlayer distance d002 were 0.3385 nm for the fiber with a diameter of about 0.15 μm, 0.3400 nm for the fiber with a diameter of about 0.05 μm, 0.3415 nm for the fiber with a diameter of about 0.02 μm, and 0.3420 nm for the fiber with a diameter of about 0.01 μm or less. Therefore, the limitation of the interlayer space d002 is 0.3385 nm and, even when the carbon fibers are heated to 3000° C. or higher, d002 cannot be reduced to less than 0.3385 nm.
Accordingly, in order to enhance crystallinity and to reduce d002 to 0.3385 nm or less, there must be developed a method for enhancing crystallinity other than heat treatment.
The object of the present invention is to develop fine carbon fibers having a high crystallinity that has not been produced through a conventional method or having a high crystallinity, and to provide a battery electrode having a higher performance by containing the developed carbon fibers as a filler.